Simulation of Penobscot Bay Circulation

 

Penobscot Bay is the largest estuarine embayment in the Gulf of Maine, the second largest on the U.S. east coast. The Bay has been historically and remains a very important fishery ground. It harbors the most productive lobster habitat and accounts for roughly 50% of the lobster landings for the entire state of Maine. Penobscot Bay also supports an extensive tourism industry, and acts as an active commercial ship traffic conduit. In short, the socio-economic importance of Penobscot Bay extends far beyond its immediate communities.

Knowledge of the circulation in Penobscot Bay is fundamental to the understanding of the Bay as an ecological system since the circulation constitutes the framework within which the entire system operates. For example, circulation controls the transport of planktonic and fish larvae impacting recruitment success, and it also controls the transport and dispersion of pollutants. Circulation models are powerful tools in providing comprehensive pictures of the physical environment of the Bay. Oceanographic observations are costly. Therefore, it is not possible to frequently sample a bay like Penobscot with many stations. Remote sensing data provides surface measurements, but are hampered in cloudy days, and high spatial resolution images (e.g. LANDSAT) are much less frequent. On the other hand, a numerical model is able to fill data gaps if the model can reasonably reproduce the observed data, and the model capability of reproducing particular oceanic features (e.g., fronts and eddies) can be improved substantially by assimilating satellite derived information (SSH and/or SST) in combination with some sort of in situ measurements (A Real-Time Nowcast/Forecast System for the Gulf of Mexico. Another important aspect is the understanding of the circulation dynamics at a predictive level. The potential of being able to predict with reasonable certainty and lead times is well justified in policy making and management.

A comprehensive ocean circulation model (the Princeton Ocean Model) is applied to Penobscot Bay and the adjoining shelf water off the mid-coast of Maine. The model has 151 x 121 horizontal grid points (Fig. 1) and 15 sigma levels (Fig.2). Six stations are chosen and marked on Fig. 1 where time series of modeled salinity, temperature, u and v velocities are shown later. Two bathymetry data sets are used in this study. Fig. 3a is based on the NGDC data set, Fig. 3b uses the larger depth of NGDC or USGS data set at any given point. Minimal and maximum water depth are 2 m and 145 m, respectively.

 

Model Simulation of March - September, 1997

The first experiment is forced with daily Penobscot River discharge rate and the hourly wind at Matinicus Rock (Fig. 4a) during the months of April and May 1997. Net heat and fresh water fluxes are obtained from the COADS monthly climatology (Fig. 4b). Time series of salinity (Fig. 5a and 5b), temperature (Fig. 5c and 5d), u-velocity (Fig. 5e and 5f), and v-velocity (Fig. 5g and 5h) at six stations suggest obvious correlation with wind events.

The second experiment is forced with daily Penobscot River discharge rate and the hourly wind at Matinicus Rock (Fig. 6a) from March to September 1997. The magnitude of the wind decreases from offshore to inside of the Bay. Net heat and fresh water fluxes are again from the COADS monthly climatology (Fig. 6b). Time series of salinity, temperature, u and v at six stations are shown in Fig. 7a, 7b, 7c, 7d, 7e, 7f, 7g, and 7h. Differences during the concurrent period (April-May) are very small.

 

Response of the Bay during two particular wind events

1. Northeasterly wind event around day 18

a) surface salinity and velocity

b) salinity and velocity at 30 m depth

c) velocity, salinity, and temperature cross the lower bay

d) velocity, salinity, and temperature in the western bay

e) velocity, salinity, and temperature in the eastern bay

 

2. Southwesterly wind event around day 46

a) surface salinity and velocity

b) salinity and velocity at 30 m depth

c) velocity, salinity, and temperature cross the lower bay

d) velocity, salinity, and temperature in the western bay

e) velocity, salinity, and temperature in the eastern bay

 

Model Simulation of 1998 (April - September)

This experiment is forced with daily Penobscot River discharge rate and the hourly wind at Matinicus Rock (Fig. 8) from April to September 1998. Time series of velocities at the outer western Penobscot Bay (WPB) and the outer eastern Penobscot Bay (EPB) mooring sites are shown in Fig. 9a and 9b. Compared with the observed velocity time series at the same sites (Fig. 10a and 10b), the model reproduces the three-layer structure in the outer western bay with outflows at the surface and the bottom and inflows in the middle of the water column and the two-layer structure in the outer eastern bay with outflows at the surface and inflows in the lower water column. However, the surface outflows reach the 20 m depth at both locations which is deeper than the observed at the WPB and shallower than the observed at the EPB. We are working on sensitivity experiments of model parameterization and open boundary condition and hope to achieve a better agreement of layer thickness.

Penobscot Bay responses quickly to the synoptic wind forcing. apr98, a short movie of the modeled surface salinity and velocity for the month of April in 1998, illustrates the rich temporal and spatial variability of the bay. The curve at the top shows the Penobscot River discharge rate and the moving stick along the abscissa shows the averaged wind over the last 12 hr period.

Two instances are selected to demonstrate the differences in the circulation pattern. The first is concurrent with the April hydrographic survey so that the model results (Fig. 11 and 12) can be compared with the observation (Fig. 13). At that time, winds were from southwest and light.

The second is around June 14. The Bay was forced by strong southeasterly winds and acted rather differently (Fig.14 and 15) such that at the WPB mooring site outflows extended throughout the water column (see Fig. 9a and 10a), whereas at the EPB mooring site the surface flows changed from southward to northward and the bottom flows changed from northward to southward (see Fig. 9b and 10b).

 

M2 tide

surface elevation and vertical averaged velocity during high tide, ebbing, low tide and flooding.

Vertical profiles of u, v, vertical and horizontal mixing coefficients over a tidal cycle at station 1, 2, 3, 4, 5, and 6.

 

Under Construction!